The technique of down-the-hole (DTH) percussive hammer drilling involves the supply of a pressurised fluid via a drill string to a drill bit located at the bottom of a bore hole. The fluid acts to both drive the hammer drilling action and to flush rearwardly dust and fines resultant from the cutting action, rearwardly through the bore hole so as to optimise forward cutting.
Typically, the drill assembly comprises a casing extending between a top sub and a drill bit that, in turn, is releaseably coupled to a drive component (commonly referred to as a chuck or drive sub). Drilling is achieved via a combination of rotation and axial translation of the drill bit. Rotation is imparted to the drill bit from the drive sub via intermediate engaging splines. The axial percussive action of the bit is achieve via a piston that is capable of shuttling axially between the top sub and the drill bit and is driven by the pressurised fluid to strike a rearward anvil end of the bit. A foot valve extends axially rearward from the drill bit to mate with the piston during its forwardmost stroke to control both the return stroke and provide exhaust of the pressurised fluid from the drill head that acts to flush rearwardly the material cut from the bore face. Example DTH hammer drills are described in WO 2008/051132 and WO 2013/104470.
Conventionally, the drill bit is retained at the assembly and in contact with the drive sub via a retaining ring accommodated within the assembly. However, due to the significant loads imparted to the drill bit, it is common for the drill bit head to shear from the bit shank. This disrupts drilling operation significantly as boring cannot be resumed until the detached bit head is recovered as it otherwise represents an impenetrable barrier to forward drilling and would in turn significantly damage a replacement drill bit. GB 2385869; US 2010/0263932; WO 2009/124051 and U.S. Pat. No. 7,117,939 disclose DTH drive couplings that attempt to retain a detached or ‘shanked’ drill head at the assembly so that the head may be retracted with the drill string to avoid retrieval problems. In particular, a retaining sleeve extends axially between the drive sub and the bit head to ‘catch’ and retain the head in the event of detachment.
However, these conventional drive coupling arrangements are disadvantageous for a number of reasons. Fundamentally, as a result of the magnitude of the torque transmitted between the drive sub and the hammer casing, it is typically very difficult to remove the drive sub and ‘break-open’ the assembly without dedicated tooling that may not be available on-site. The arrangements of the prior art are typically focussed towards detached bit head retention and in turn compromise the ease and time required for removal and installation of a replacement drill bit that typically requires a partial dismantling of the assembly.
A further problem with conventional coupling assemblies is the accelerated wear of all or part of the coupling components that may in turn accelerate wear of or cause damage to other components of the assembly. In particular, the retaining sleeves described in GB 2385869 and US 2010/0263932 are rotatably locked at the drive sub. Due to the configuration of these couplings, the worn components cannot be replaced without breaking-open the drive sub and encountering the aforementioned problems. Accordingly, there exists a need for a drill bit coupling arrangement that addresses the above problems and provides for the convenient and rapid removal and installation of a replacement drill bit at the drive assembly.